UWBG (ultra-wide bandgap) semiconductor electronic devices are of current interest because of high breakdown voltages and other advantages. For example, a HEMT (high electron mobility transistor) fabricated in III-nitrides or another UWBG material system has a theoretical on-resistance, at given breakdown voltages, greater than what other technologies offer.
More specifically, a lateral figure of merit (LFOM) has been defined for lateral power devices whose performance may be limited by conduction losses:
  LFOM  =            q      ⁢                          ⁢      μ      ⁢                          ⁢              n        s            ⁢              E        C        2              =                            V          br          2                          R                      on            ,            sp                              .      
In the above equation, p is the electron mobility, q is the electronic charge, ns is the sheet charge, EC is the critical field for avalanche breakdown, Vbr is the off-state breakdown voltage, and Ron,sp is the specific on-state resistance. A discussion of the LFOM may be found, for example, in Jordan D. Greenlee et al., ECS J. Solid-State Sci. Technol. 4, P382 (2015).
The LFOM favors semiconductor materials with large bandgap, such as the AlGaN alloys and AlN, particularly at high junction temperatures, because the breakdown voltage tends to increase with increasing bandgap. The LFOM also favors smaller values of the specific on-resistance. Reducing the on-resistance at a given operating voltage (as limited by breakdown) is beneficial for applications in power electronics because it can lead to greater efficiency and smaller size, power and weight.
The material parameters limit the maximum theoretical value that the LFOM can reach. In practice, however, the LFOM may be limited by a sub-optimal Vbr or Ron,sp. Hence in order to optimize the device performance, it is desirable to design for optimal values of these parameters.
For example, it has proven difficult to provide low-resistance Ohmic contacts for devices fabricated in some material systems. The advantage of LFOM promised by UWBG materials will be unrealized if the Ohmic contacts for the source and drain contact regions have excessively high resistance.
Known approaches in the context of AlxGa1-xN/AlyGa1-yN HEMTs involve judicious selection of the metals to optimize conventional Ohmic metal stacks, as well as selective doping of the Ohmic contact regions by ion implantation. However, contact resistivities still remain substantially greater than those typical for GaN and other semiconductors.
Another challenge in UWBG design is that power electronics systems are often required to shut down by default when power to the control circuit is lost. This dictates the use of normally-off, or enhancement-mode, power transistors in which a zero voltage applied to the gate will leave the device in its off condition.
More specifically, electrical conduction in a HEMT depends on the existence of a two-dimensional electron gas (2DEG) near the interface between a channel layer and a barrier layer. The enhancement-mode condition will deplete the charge in the 2DEG, resulting in zero conductivity between the source and the drain. A positive gate voltage will restore conductivity to the channel region under the gate.
There are known techniques for enhancing conductivity in the so-called access regions of a HEMT that lie between the source and the gate and between the gate and the drain. However, not all such techniques are suitable for optimizing the product μ·ns in an enhancement-mode channel, as would be required in order to achieve maximum LFOM in an enhancement-mode device.
One such technique is self-aligned ion implantation and annealing of the source-gate and the gate-drain regions to enhance conductivity in those regions. However, ion implantation does not work well in III-nitride materials, and it needs further development before it can receive widespread acceptance.
Another technique, gate recessing, is often used in other III-V material systems such as GaAs. However, gate recessing has limited value when applied to the III-nitride materials because the chemical reactivity of these materials tends to be relatively low, with the consequence that a suitable wet etch for low-damage recessing has not been adopted. Performing a dry etch for gate recessing is feasible, but it can lead to gate leakage and reduced breakdown voltage.